In recent years, there is a growing demand for saving fuel of an automobile due to a global issue of carbon dioxide emission and worldwide increase of energy demand. Under these circumstances, it is more demanded than before to improve a power transmission efficiency of a transmission, and it is also demanded to achieve a high torque capacity of the lubricating oil that is a major constituent component.
Lowering a viscosity of the lubricating oil may be an example as a means for saving fuel of the transmission. Among the transmission, an automatic transmission and a continuously variable transmission for automobiles have a torque converter, a wet clutch, a gear bearing mechanism, an oil pump and a hydraulic control system. Lowering the viscosity used in these transmissions reduces agitation- and frictional resistance, thereby improving the power transmission efficiency to improve fuel consumption of the automobiles.
However, the lubricating oil having the lowered viscosity increases the influence of contact of metals, so that a fatigue life of a machine component such as a bearing and a gear is considerably reduced to cause some malfunctions in the transmissions and the like.
Lubricating oil compositions for transmissions having a long fatigue life while keeping a low viscosity are disclosed in Patent Documents 1 to 4.
Patent Document 1: JP-A-2006-117851
Patent Document 2: JP-A-2006-117852
Patent Document 3: JP-A-2006-117853
Patent Document 4: JP-A-2006-117854